Metal particles obtained by processing gold, silver, nickel and the like into particle forms have been used as conventional conductive fine particles, but they have a large specific weight and inconsistent forms, and therefore, these particles sometimes fail to disperse uniformly in a binder resin, causing inconsistent conductivity in the anisotropic conductive material.
In contrast, conductive fine particles where the surface of nonconductive particles, such as glass beads, glass fibers or plastic balls, which are used as core particles, are plated with a metal, such as nickel, have been known. Patent Document 1, for example, discloses conductive electroless plated powders where a metal coating is formed on resin powder particles which are substantially spherical by an electroless nickel plating method.
In the metal coating film resulting from such an electroless nickel plating method, however, the nickel coating film contains phosphorous, and therefore, the conductivity is low in comparison with pure nickel metal, and in addition, the conductivity of the nickel coating film tends to worsen as the phosphorous content in the nickel coating film increases.
In addition, in accordance with this electroless nickel plating method, the surface area of the core particle itself is large, making a cohesive force large, and therefore, it is difficult to precisely control the rate of plating reaction in a method for adding a nickel salt solution which forms a nickel plating solution and a solution of a reducing agent, such as sodium hypophosphite, little by little to a dispersion with a pH remaining constant, and the cohesive property of the conductive fine particles cannot sufficiently be suppressed.
Meanwhile, Patent Document 2 discloses conductive fine particles where a metal coating layer made of nickel or cobalt and containing 1.5 to 4 wt % of phosphorous is provided on the surface of resin fine particles. Furthermore, Patent Document 3 discloses nickel plated particles where the content of phosphorous in the nickel coating film varies in the direction of the thickness of the nickel coating film, in such a manner that the content gradually becomes lower from the core particle side to the surface side of the nickel coating film, so that the conductivity is not lost.    Patent Document 1: Japanese Kokai Publication Hei-8-311655    Patent Document 2: Japanese Patent Publication 2507381    Patent Document 3: Japanese Kokai Publication 2003-34879